Monitoring certain parameters of left ventricular function can provide information useful for evaluating a patient's condition during surgical procedures. These parameters also provide information that can be used to detect coronary heart disease and other medical problems of the heart. One of the most commonly used parameters for diagnostic purposes is the left ventricular global ejection fraction, which expresses the proportion of chamber volume ejected with each heart beat. Other important parameters include the range of motion of the left ventricular wall and the thickening of the ventricular wall, both of which are indicators of coronary heart disease, and of other disease entities.
The effects of coronary heart disease are regional, being limited to the portion of the heart muscle receiving blood supply from an affected artery. When the internal diameter of an artery is reduced by atherosclerotic plaque, blood flow to the specific region of the heart supplied by that artery is restricted. As a result, some degree of dysfunction occurs in the affected heart muscle. During a heart attack, the affected muscle dies and is replaced by scar tissue, which is non-contractile. Thus, the progress of coronary artery disease is revealed by its effect on regional left ventricular function, and the severity of a heart attack is measured by the size of the dysfunctioning region and by the extent of the dysfunction. Similarly, any improvement of regional function in the affected portion of the left ventricle is an indication of the effectiveness of a prescribed treatment.
The appearance of dysfunction in a previously well-functioning ventricle is a serious warning that the blood supply is insufficient. Should a deterioration of function occur during surgery, it may be construed as an indication that the anesthesiologist should increase the fluid volume and/or engage in other corrective measures.
The detection of regional dysfunction has also been used during stress studies, wherein a patient's heart is imaged using ultrasound while at rest and after exercise, to determine whether the patient's arteries, which may have been open sufficiently while at rest, provide inadequate blood flow during exercise. The degree of dysfunction after a heart attack has occurred may also be determined to develop a prognosis. For example, patients with serious residual dysfunction after a heart attack are at a higher risk of dying in the first year and more aggressive treatment may be indicated.
As noted above, one of the preferred methods to detect and evaluate regional dysfunction in the left ventricle is to measure the range of wall motion, i.e., the range of movement of the ventricular wall during a cardiac cycle. Another approach is to measure regional wall thickening, which is also an indication of coronary disease and muscle dysfunction. Previous techniques for measuring these parameters have typically relied upon two-dimensional imaging of the cardiac wall, which can introduce significant error due to failure to compensate for the angle of the beam relative to the cardiac wall. In addition, two-dimensional imaging generally is limited in its ability to clearly localize regional left ventricular dysfunction or provide an overall view of an affected region that allows a physician to immediately interpret the extent and degree of dysfunction. Prior art attempts to model the heart in three-dimensional views have not been entirely successful, because such attempts have been based on spherical or helical coordinate systems that can not accurately determine minimum wall thickness in a specific region. Such coordinate system based attempts may be unsuitable in patients whose hearts are distorted by disease. Furthermore, prior art three-dimensional imaging and modeling techniques have been limited to the left ventricle, which tends to be more regular and consistent in size and shape in different patients; in contrast, other portions of the heart exhibit greater variation in size and shape that has precluded referential modeling of such portions for comparison to the corresponding portion of normal, undiseased hearts. Accordingly, it will be evident that a new approach to monitoring cardiac parameters, such as range of wall motion and wall thickening, is needed that is more reliable and provides greater resolution and accuracy in identifying problem regions in the heart.
More importantly, it would be desirable to achieve the improved measurement of such cardiac parameters in real time, so that the technique can be employed during open heart surgery to continuously evaluate the condition of a patient's heart and the effect of anesthesia on the patient. An optimum device for imaging selected portions of the heart is a transesophageal ultrasonic probe. With an imaging probe disposed in the esophagus behind the heart, most of the heart structure can be imaged without interference from the lungs or ribs, as often occurs with transcutaneous imaging. If the patient is anaesthetized during the procedure, there is no discomfort and the probe can be maintained at the same position for several hours. A method for using the image provided by such a probe to model a selected region of the heart so as to determine range of cardiac wall motion and wall thickness at specific points on the cardiac wall in real time will therefore be of significant benefit during cardiac surgery. Furthermore, a method that references specific regions of the heart to a standard or average cardiac template so as to provide a specific identification of an affected region that is generally independent of the size and shape of a patient's heart (even if abnormal due to disease) should aid in better assessing problems that are diagnosed.